This invention relates generally to the production of viscous hydrocarbons from subterranean hydrocarbonscontaining formulations. More specifically, it relates to the control of flowing viscosity of produced fluids within a wellbore. Deposits of highly viscous crude petroleum represent a major future resource in the United States in California and Utah, where estimated remaining-in-place reserves of viscous or heavy oil are approximately 200,000,000 barrels. Overwhelmingly, the largest deposits in the world are located in Alberta Province, Canada, where the in-place reserves approach 1,000 billion barrels from depths of about 2,000 feet to surface outcroppings and occurring at viscosities in excess of 1,000,000,000 cp at reservoir temperature. Until recently, the only method of commercially recovering such reserves was through surface mining at the outcrop locations. It has been estimated that about 90% of the total reserves are not recoverable through such mining operations. U.S. Pat. No. 4,037,658 to Anderson teaches a method of assisting the recovery of viscous petroleum, such as from tar sands, by utilizing a controlled flow of hot fluid in a flow path within the formation but out of direct contact with the viscous petroleum; thus, a solid-wall, hollow, tubular member in the formation is used for conducting hot fluid to reduce the viscosity of the petroleum to develop a potential passage in the formation outside the tubular member into which a fluid is injected to promote movement of the petroleum to a production position.
The method and apparatus disclosed by the Anderson '658 Patent and related patents is effective in establishing and maintaining communications within the producing formation, and has been termed the "Heated Annulus Steam Drive," or "HASDrive method." In the practice of HASDrive, a hole is formed in the petroleum-containing formation and a solid-wall, hollow, tubular member is inserted into the hole to provide a continuous, uninterrupted flow path through the formation. A hot fluid is flowed through the interior of the tubular member out of contact with the formation to heat viscous petroleum in the formation outside the tubular member to reduce the viscosity of at least a portion of the petroleum adjacent the outside of the tubular member to provide a potential passage for fluid flow through the formation adjacent the outside of the tubular member. A drive fluid is then injected into the formation through the passage to promote movement of the petroleum for recovery from the formation.
Parallel tubing strings, the apparatus disclosed in U.S. Pat. No. 4,595,057 to Deming et al, is a configuration which at least two tubing strings are placed parallel in the wellbore casing. Parallel tubing has been found to be superior in minimizing scaling and heat loss during thermal well operation.
U.S. Pat. No. 5,014,787 which is assigned to the assignee of the present application and incorporated herein by reference, achieves an improved heavy oil recovery from a heavy oil containing formation utilizing a multiple tubing string completion in a single wellbore, such wellbore serving to convey both injection fluids to the formation and production fluids from the formation. The injection and production would optimally occur simultaneously, in contrast to prior cyclic steaming methods which alternated steam and production from ,a single wellbore. The process disclosed in U.S. Pat. No. 5,014,787 394 is termed the "Single Well Injection/ Production System," or "SWIPS." In the SWIPS process, it is not necessary the wellbore be substantially horizontal relative to the surface but may be at an any orientation within the formation. By forming a fluid barrier within the wellbore between the terminus of the injection tubing string and the terminus of the production tubing string; and exhausting the injection fluid near the barrier while injection perforations are nearer the wellhead, the SWIPS wellbore casing is effective in mobilizing at least a portion of the heavy oil and the formation nearest the casing by conduction heat transfer.
The improved heavy oil production method disclosed by U.S. Pat. No. 5.014,787 is thus effective in establishing communication between the injection zone and production zone through the ability of the wellbore casing to conduct heat from the interior of the wellbore through the heavy oil in the formation near the wellbore. At least a portion of the heavy oil in the formation near the wellbore casing would be heated, its viscosity lowered and thus have a greater tendency to flow. The single well method and apparatus of the SWIPS method and apparatus in operation therefore accomplishes the substantial purpose of an injection well, a production well, and a means of establishing communication therebetween.
Of great concern in the production of viscous hydrocarbons is the potential for flowing hydrocarbons within a tubular member to so cool in temperature as to effectively cease flowing and therefore inhibit further production. Without a means for elevating the temperature of such lowered temperature viscous hydrocarbons within a tubular flow path, viscous hydrocarbon production would be jeopardized. While the oil produced from the hydrocarbon bearing formation is capable of flowing at an elevated temperature, if allowed to cool the fluid viscosity would drastically increase, and production of oil greatly inhibited. One method of insuring the fluid within the production tubing is maintained at a desired elevated temperature is to exhaust a portion of hot injection fluid from the injection tubing to the annulus formed between the casing and the tubing strings and thus conduct heat through the production tubing wall to the produced fluid within the production tubing. By the method of the present invention, a subsurface flow controlled device, such as the "Control-A-Flow Sliding Side Door.RTM." device manufactured by Otis Engineering, or the like, is placed within the injection tubing just above the dual packer. A portion of hot injection fluid would thus be allowed to conduct heat to the production tubing prior to that portion of hot injection fluid being exhausted from the wellbore annulus at the surface. When a desired flowing temperature is achieved in the production tubing, the subsurface flow control device may be closed, and normal injection and production operations in accordance with the SWIPS method resumed.